JP2009222397A - Optical fiber sensor and measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To take out only distortion by eliminating the effect of a temperature change generated in an FBG sensor. <P>SOLUTION: A fiber support 21 of an optical fiber sensor 20 is erected so that a pair of arm sections 25, 26 opposes each other via hinges 23, 24 at one surface side of a base 22 fixed to a measurement target; the middle positions of the arm sections 25, 26 are connected by a connection section 29 via hinges 27, 28; the fiber support is formed integrally by a material having an expansion coefficient equal to that of the measurement target; and the arm sections 25, 26 are tilted in opposite directions mutually by receiving force for stretching the base 22 from the measurement target. Both the ends of the optical fiber 1 for sensors having a grating section 1a are fixed with tension between positions at one end side separated from the connection section 29 of the arm sections 25, 26 by a prescribed distance; and an optical fiber 1' for sensors having the same characteristics as those of the optical fiber is fixed being balanced between positions at the other end side separated from the connection section 29 of the arm sections 25, 26 by a prescribed distance. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical fiber sensor for detecting strain generated in a measurement object using an FBG (fiber Bragg grating) type optical fiber having a grating portion formed in a core, and the influence of temperature on the output of the optical fiber sensor. It is related with the technique for suppressing.

  It occurs in the structure and machinery in order to grasp and diagnose the durability, fatigue, damage, life, etc. (hereinafter collectively referred to as soundness) of machinery such as buildings and bridges and equipment. Measurement of strain distribution and strain history (strain progress rate, maximum strain amount, change in dynamic characteristics, etc.) may be required.

  In soundness monitoring in a conventional structure, electrical strain gauges and acceleration sensors are attached to the required parts of the structure (for example, stress-concentrated parts such as steel welds and joints and reinforcing vertical brace materials). Is measured.

  However, electrical sensors such as strain gauges require various types of processing such as electromagnetic noise / lightning countermeasures and rust prevention processing in the sensor section and signal transmission section. The health monitoring used is attracting attention. Since the optical fiber sensor uses an optical signal, it is resistant to electromagnetic noise, has explosion-proof properties, does not require rust-proofing, has durability that can be used outdoors for a long time, and is suitable for soundness monitoring.

  One of the optical fiber sensors for the above uses is an FBG (Fibber Bragg Grating) type (hereinafter, this optical fiber sensor is referred to as an FBG sensor).

  This FBG sensor is a grating in which the refractive index of the core portion of the optical fiber is periodically changed in the light traveling direction.

As shown in the following formula (1), the grating part (sensitive part) of the FBG sensor is a light having a Bragg wavelength λ _B determined by the refractive index change period d and the effective refractive index n ₀ and a wavelength in the vicinity thereof. Only reflects and transmits light of other wavelengths.

λ _B = 2n ₀ d (1)

From equation (1), it can be seen that the Bragg wavelength λ _B reflected from the grating portion also changes when the refractive index n ₀ or the period d of the grating portion changes.

For example, when an external force is applied to the grating portion of the FBG sensor and a strain ε is generated in the axial direction of the optical fiber, the grating portion expands and contracts accordingly, the period d changes, and the effective refractive index n ₀ also changes. To do. The shift amount Δλ _B of the Bragg wavelength caused by the change in the period and the refractive index is given by the following equation (2).

Δλ _B1 ≈λ _B · (1−ρ _C ) · ε (2)
However, ρ _C is a photoelastic coefficient and is an amount representing an effective refractive index change of the grating portion due to the strain ε.

From the above equation (2), if the Bragg wavelength shift amount Δλ _B1 is detected after fixing the grating portion of the FBG sensor to the measurement target, the strain ε of the measurement target with the grating Bragg wavelength fixed can be measured. it can.

However, the period d and the effective refractive index n ₀ of the grating part of the FBG sensor change not only by an external force but also by a temperature change ΔT. Equation (3) is to determine the relationship between temperature change ΔT and the shift amount [Delta] [lambda] _B2 by partially differentiating both sides of the equation (1) at a temperature.

Δλ _B2 = λ _B ζΔT + λ _B αΔT (3)
Here, ζ is the rate of change of period d associated with temperature change ΔT, and α is the rate of change of refractive index n ₀ associated with temperature change ΔT.

  Therefore, the shift amount of the Bragg wavelength when the external force and the temperature change ΔT are simultaneously loaded on the grating portion of the FBG sensor is expressed as the following equation (4).

Δλ _fbg = Δλ _B1 + Δλ _B2 (4)

That is, the shift amount Δλ _fbg of the Bragg wavelength actually detected by the FBG sensor is a result including the influence of the external force and the temperature change, in order to detect only the shift amount Δλ _B1 due to the strain ε generated in the measurement target. Requires temperature compensation to remove the shift amount Δλ _B2 due to temperature change.

  As a conventional temperature compensation method for an optical fiber sensor, in Patent Document 1 below, as shown in FIG. 8, a strain measurement FBG sensor 61 is fixed on a substrate 63 having a linear expansion coefficient equal to that of an optical fiber, A method is proposed in which the FBG sensor 62 for temperature compensation is fixed in a relaxed state so that the displacement is not constrained on the substrate 63, and the strain ε is measured by the difference in the shift amount of the Bragg wavelength detected by both the FBG sensors 61 and 62. Has been.

Japanese Patent No. 2983018

  Further, in the following Patent Document 2, as shown in FIG. 9, a first FBG sensor 61 and a second FBG sensor 62 are provided between a first holder 64 and a second holder 65 that are relatively movable in the X direction. Is fixed such that when one tension increases, the other tension decreases, and a method of measuring the strain ε in the X direction based on the difference in the Bragg wavelength shift detected by both FBG sensors 61 and 62 is disclosed.

JP 2002-257520 A

  Furthermore, in Patent Document 3 below, as shown in FIG. 10, both ends of the strain measurement FBG sensor 61 are fixed to the measurement object 50 by the attachment plates 67 and 68, and the thermal expansion coefficient is equal to that of the measurement object 50. A method is disclosed in which only one end of the temperature compensating FBG sensor 62 is fixed to the measurement object 50 with the plate 69 and the strain ε is measured by the difference in the amount of shift of the Bragg wavelength detected by both the FBG sensors 61 and 62.

Japanese Patent No. 3711905

  However, the temperature compensation methods disclosed in the above Patent Documents 1 to 3 cannot remove the influence of all temperature changes from the shift amount of the Bragg wavelength actually detected by the FBG sensor, so that the distortion of the measurement target is accurate. There is a problem that cannot be measured well.

In general, as shown in FIG. 8, the FBG sensor is fixed to the surface of the measurement object 50 after being attached to a fixing plate such as a substrate 63 with an adhesive or the like. When the thermal expansion coefficient of the fixed plate is different from that of the FBG sensor, the expansion and contraction of the fixed plate accompanying the temperature change ΔT is transmitted to the FBG sensor, and the influence of the expansion and contraction of the fixed plate is reflected on the shift amount of the Bragg wavelength. When the thermal expansion coefficient of the measurement target 50 is different from that of the fixed plate, the expansion / contraction of the measurement target 50 due to the temperature change ΔT is transmitted to the FBG sensor through the fixed plate, and the shift amount of the Bragg wavelength is included in the shift amount of the measurement target 50. The effect of expansion and contraction is reflected. Therefore, in order to perform temperature compensation of the shift amount of the Bragg wavelength actually detected by the FBG sensor, not only the influence (Δλ _B2 ) caused by the temperature change ΔT on the FBG sensor itself, but also a fixed plate connected to the FBG sensor It is necessary to remove the effect of expansion and contraction due to temperature changes of the measurement target.

Specifically, if the linear expansion coefficient α of the FBG sensor and the linear expansion coefficient β of the fixed plate are different, the FBG sensor receives a strain equal to (β−α) ΔT due to expansion and contraction of the fixed plate accompanying the temperature change ΔT. The influence of the expansion and contraction of the fixed plate is the shift amount Δλ _B3 of the Bragg wavelength λ _{B in} the following equation (11), and the shift amount Δλ sensor due to the influence of the detected temperature change ΔT is expressed by the equation (12).

Δλ _B3 = λ _B (1−ρ _C ) (β−α) ΔT (11)
Δλsensor = Δλ _B2 + Δλ _B3 (12)

  When the linear expansion coefficient β of the fixed plate and the linear expansion coefficient γ of the measurement target 50 are different, the FBG sensor is changed to (γ−β) ΔT via the fixed plate due to the expansion and contraction of the measurement target 50 due to the temperature change ΔT. Receive equal strain. The influence of the expansion / contraction of the measurement object 50 becomes the shift amount of the Bragg wavelength in the following equation (13), and the shift amount Δλtarget due to the influence of the detected temperature change ΔT is expressed by the equation (14). In the equation (13), η is a strain magnification ratio, which indicates a ratio of an effective strain transmitted to the FBG sensor among strains generated in the fixed plate due to expansion / contraction of the measurement target 50, and is determined by the structure of the fixed plate. Is a coefficient.

Δλ _B4 = λ _B η (1-ρ _C ) (γ−β) ΔT (13)
Δλtarget = Δλ _B2 + Δλ _B3 + Δλ _B4 (14)

On the other hand, the strain ε of the measurement object 50 caused by the external force is also transmitted to the FBG sensor via the fixed plate. The shift amount Δλ _B5 due to the influence of the external force is expressed by the following equation (15) using the strain magnification factor η. When the external force and the temperature change ΔT are simultaneously applied, the shift amount Δλall in Expression (16) is detected.

Δλ _B5 = λ _B η (1-ρ _C ) ε (15)
Δλall = Δλ _B2 + Δλ _B3 + Δλ _B4 + Δλ _B5 (16)

  Therefore, in order to accurately measure the strain ε of the measurement target 50 with the FBG sensor, it is necessary to remove the shift amount Δλtarget of the equation (14) due to the influence of the temperature change ΔT from the detected shift amount Δλall of the equation (16). There is.

On the other hand, in the temperature compensation method shown in FIG. 8, the strain measurement FBG sensor 61 detects the shift amount Δλall of the equation (16), and the difference between the shift amounts of the FBG sensors 61 and 62 is (Δλ _B3 + Δλ _B4 + Δλ _B5 ). Here, since the linear expansion coefficient α of the FBG sensor and the linear expansion coefficient β of the fixed plate (substrate 63) are equal, the shift amount Δλ _B3 of the equation (11) is 0, but as in the following equation (21): In addition to the shift amount to be obtained, the term Δλ _B4 (= λ _B η (1−ρ _C ) (γ−β) ΔT) depending on the temperature change ΔT remains, and the influence of the temperature change ΔT can be removed. Can not. Further, in the method of FIG. 8, the temperature compensating FBG sensor 62 is held in a relaxed hollow state, so that the temperature compensating FBG sensor 62 is easily deformed in response to vibration applied from the outside, and during a long-term measurement period. The measurement accuracy of the strain ε may be deteriorated.

Δλall −Δλ _B2 = Δλ _B4 + Δλ _B
= Λ _B η (1−ρ _C ) (γ−β) ΔT + λ _B η (1−ρ _C ) ε (21)

In the temperature compensation method shown in FIG. 9, when the first holder 64 and the second holder 65 are relatively moved, a reverse force is applied to the first FBG sensor 61 and the second FBG sensor 62. The influence of the strain ε due to the external force detected by both the FBG sensors 61 and 62 (shift amount Δλ _{B5 in} Expression (15)) is opposite in sign. Also. The sign of the influence of the expansion and contraction of the measurement object 50 due to the temperature change ΔT (shift amount Δλ _{B4 in} equation (13)) is also reversed. That is, the first FBG sensor 61 detects the shift amount Δλall ₍₊ ) of the equation (16), whereas the second FBG sensor 62 detects the shift amount Δλall _{(−) in which} the signs of Δλ _B4 and Δλ _B5 are reversed. (= Δλ _B2 + Δλ _B3 −Δλ _B4 −Δλ _B5 ) is detected. Therefore, the difference between the shift amounts of the FBG sensors 61 and 62 is expressed by the following equation (22), which is also a term 2Δλ _B4 (= 2λ _B η (1-ρ _C ) (γ−β) ΔT) that depends on the temperature change ΔT. Therefore, it is impossible to extract only the component Δλ _B5 of the strain ε of the measurement target 50 to be obtained.

Δλall ₍₊₎ −Δλall ₍₋₎ = 2Δλ _B4 + 2Δλ _B5
= 2λ _B η (1−ρ _C ) (γ−β) ΔT + λ _B η (1−ρ _C ) ε (22)

Further, in the temperature compensation method shown in FIG. 10, the linear expansion coefficient β of the fixing plate 69 of the temperature measurement FBG sensor 62 is made equal to the linear expansion coefficient γ of the measurement target 50, so that it is detected by the FBG sensor 62. The influence of the strain due to the expansion and contraction of the fixed plate is a shift amount Δλ _B3 ′ of the following equation (23). That is, the strain measuring FBG sensor 61, while the shift amount Δλall of formula (16) is detected, the temperature compensating FBG sensor 62, the shift amount [Delta] [lambda] _B2 of formula (3) due to the influence of the temperature change ΔT A shift amount Δλ sensor (= Δλ _B2 + Δλ _B3 ′) obtained by superimposing the shift amount Δλ _B3 ′ in Expression (23) is detected. Therefore, the difference between the shift amounts of the two FBG sensors 61 and 62 is expressed by the following equation (24), and the term (η−1) (1-ρ _C ) (γ−α) ΔT depending on the temperature change ΔT remains. Therefore, the strain ε of the measurement object 50 cannot be measured with high accuracy. In addition, since the temperature measurement module has a cantilever structure, the temperature compensation FBG sensor 62 may be deformed during a long measurement period and the measurement accuracy may deteriorate.

Δλ _B3 ′ = λ _B η (1−ρ _C ) (γ−α) ΔT (23)
Δλall−Δλsensor
= (Η-1) (1-ρ _C ) (γ-α) ΔT + λ _B η (1-ρ _C ) ε (24)

  The term that depends on the temperature change ΔT, that is, the first term in the equations (21), (22), and (24) is a big problem if it is a short-term strain measurement that can ignore the temperature change ΔT. However, it becomes a big problem in the case of soundness monitoring that measures small distortion of a structure over a long period of time.

  An object of the present invention is to provide an optical fiber sensor that solves the above-described problems and can remove only the influence of the temperature change ΔT generated in the FBG sensor and extract only the strain.

In order to achieve the above object, an optical fiber sensor according to claim 1 of the present invention comprises:
A base (22) fixed to a measurement object, and a pair of arms (25, 26) standing upright so as to face each other via hinges (23, 24) which are elastically deformable on one side of the base; And a connecting portion (29) for connecting intermediate positions of the pair of arm portions via elastically deformable hinges (27, 28), and is integrally formed of a material having an expansion coefficient equal to that of the measurement object. And a fiber support body (21) that receives a force for expanding and contracting the base from the measurement target and tilts the pair of arm portions in opposite directions,
A state in which a grating part (1a) having a fiber Bragg grating is formed, and the grating part is sandwiched between positions of one end side of the pair of arm parts of the fiber support that are separated from the connecting part by a predetermined distance; A first optical fiber for sensor (1) fixed to the arm with tension at
A grating portion (1a) in which a fiber Bragg grating is formed, and the first arm portion of the fiber support body is positioned between the other end side of the predetermined distance away from the coupling portion; The tilting of the pair of arm portions due to the tension of the sensor optical fiber is restricted, and is fixed to the arm portion in a state where a tensile force for balancing the tension is applied, and is the same as the first sensor optical fiber And a second optical fiber (1 ′) for the sensor.

Moreover, the measuring device according to claim 2 of the present invention comprises:
An optical fiber sensor (20) according to claim 1;
Reflected light wavelength detecting means (41) for detecting the wavelength of the reflected light by the respective grating portions by making light incident on the first sensor optical fiber and the second sensor optical fiber of the optical fiber sensor;
Wavelength change amount detecting means for detecting a change amount of the reflected light wavelength of the first sensor optical fiber during a predetermined period and a change amount of the reflected light wavelength of the second sensor optical fiber during the predetermined period, respectively. 43)
The amount of change in wavelength associated with the distortion of the measurement object by subtracting the amount of change in wavelength of the first optical fiber for sensor detected by the wavelength change amount detection means from the amount of change in wavelength of the optical fiber for second sensor. Distortion component calculating means (44) for calculating

  As described above, the optical fiber sensor of the present invention is erected so as to face each other through the base (22) fixed to the measurement target and the elastically deformable hinges (23, 24) on one surface side of the base. A pair of arm portions (25, 26) and a connecting portion (29) for connecting intermediate positions of the pair of arm portions via elastically deformable hinges (27, 28), It has a fiber support (21) that is integrally formed of a material having an equal expansion coefficient, and that inclines the pair of arms in opposite directions in response to a force to expand and contract the base from the measurement object, Both ends of the first optical fiber for sensor (1) having the grating portion are fixed with tension between the positions of one end side of the pair of arm portions of the fiber support that is separated from the connecting portion by a predetermined distance. The optical sensor for the first sensor A second sensor optical fiber (1 ') having the same characteristics as the bar is fixed in a balanced state between the pair of arms of the fiber support at the other end side away from the connecting portion by the predetermined distance. ing.

  For this reason, the absolute value of the wavelength change amount of the reflected light by the grating portions of the optical fibers for both sensors is the same for the strain to be measured, and the direction of the wavelength change is reversed. The absolute value of the wavelength change amount of the reflected light by the grating portion of the optical fiber and the direction of the wavelength change are equal. Therefore, it becomes possible to accurately detect the distortion component to be measured in a state where the influence of the temperature change is removed by subtracting the wavelength change amounts of the optical fibers for both sensors.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a structure of an optical fiber sensor 20 to which the present invention is applied and a measuring apparatus 40 using the structure.

  The fiber support 21 of the optical fiber sensor 20 is fixed on a measurement object and expands and contracts by receiving the force of the displacement, and elastically deformable hinges 23 and 24 are provided on the upper surface side of the base 22. And a connecting portion 29 for connecting between a pair of arm portions 25 and 26 erected substantially parallel so as to oppose each other and intermediate positions of the arm portions 25 and 26 via hinges 27 and 28 that can be elastically deformed. And have. The fiber support 21 is integrally formed of a material such as a metal, a semiconductor, or a synthetic resin having the same expansion coefficient as that of the measurement target, and the base 22, the arm portions 25 and 26, and the connecting portion 29 are made of fiber. The hinges 23, 24, 27, and 28 are rigid enough not to be deformed by tension. The hinges 23, 24, 27, and 28 can be elastically deformed in a plane along a plane including the pair of arm portions 25 and 26 and the connecting portion 29. The shape is also restricted to be deformed in a direction perpendicular to the direction (thickness direction of the fiber support 21).

  The grating portion 1a of the first optical fiber for sensor 1 in which the fiber Bragg grating is formed is positioned between the arm portions 25 and 26 at one end side (the lower end in FIG. 1) at a predetermined distance L from the connecting portion 29. At two points sandwiching the grating portion 1a, it is fixed with tension (pretension) Te.

  Further, between the positions on the other end side (upper end in FIG. 1) of the arm portions 25 and 26 that are separated from the coupling portion 29 by a predetermined distance L, the base 22 has the same characteristics as the first sensor optical fiber 1. The first sensor optical fiber 1 is applied to the first sensor optical fiber 1 in a state in which no external force is applied, the arm portions 25 and 26 stand vertically to the base 22, and the connecting portion 29 is parallel to the base 22. The second sensor optical fiber 1 ′ is fixed, which applies a tension Te equal to the pretension Te and restricts the tilting of the arm portions 25 and 26 due to the pretension Te applied to the first sensor optical fiber 1. ing. Here, the optical fibers 1 and 1 'for both sensors are fixed to the arm portions 25 and 26 by the adhesive 31. However, when each fiber surface is metalized, the arm portions 25 and 26 are used. It can also be fixed by soldering.

  In this way, in the link structure in which the intermediate positions of the arm portions 25 and 26 are connected by the connecting portion 29 via the hinges 27 and 28, as shown in FIG. Both the moment and the counterclockwise moment are in an equilibrium state at L · Te.

  The measuring device 40 includes a reflected light wavelength detection means 41 and a calculation unit 42 in addition to the fiber sensor 20 having the above structure.

  The reflected light wavelength detecting means 41 is incident on one end side of the two sensor optical fibers 1 and 1 'via a light circulator from a broadband light source, for example, and is reflected by the grating portion 1a of the sensor optical fibers 1 and 1', respectively. The reflected light that has returned to one end is incident on a wavelength measuring instrument via an optical circulator and the wavelength is detected, or the light that has been wavelength-swept by a variable wavelength light source is passed through an optical circulator. 1, 1 'is incident on one end side of the light, reflected by the grating section 1a and returned to the one end side, enters the light receiver through the optical circulator, and the intensity of incident light on the light receiver reaches a peak. A configuration for detecting the sweep wavelength of the variable wavelength light source at the timing can be considered.

  The calculation unit 42 includes a wavelength change amount detection unit 43 and a distortion component calculation unit 44 in order to detect distortion of the measurement target from the wavelength of the reflected light detected by the reflected light wavelength detection unit 51.

  The wavelength change amount detecting means 43 represents the change amount of the reflected light wavelength of the first sensor optical fiber 1 during a predetermined period and the change amount of the reflected light wavelength of the second sensor optical fiber 1 ′ during the predetermined period, respectively. To detect.

  Further, the distortion component calculation means 44 subtracts the wavelength change amount of the first sensor optical fiber 1 detected by the wavelength change amount detection means 43 and the wavelength change amount of the second sensor optical fiber 1 ′. Then, the amount of wavelength change accompanying the distortion of the measurement target is calculated.

  Here, in the state where there is no temperature change, when the external force F due to the strain of the measurement target 50 is applied to the optical fiber sensor 20 in the reference state in the direction of extending the base 22 as shown in FIG. 26, as shown in FIG. 3 (b), is inclined like a letter C, and the tension of the first sensor optical fiber 1 supported by the arm portions 25, 26 with respect to the grating portion 1a increases to Te + ΔTe, The Bragg wavelength (reflected light wavelength: the same applies hereinafter) becomes longer according to the increment ΔTe. Conversely, the tension of the second sensor optical fiber 1 ′ with respect to the grating portion 1 a decreases to Te−ΔTe, and the Bragg wavelength is shortened according to the decrease ΔTe.

  Further, as shown in FIG. 4A, when the external force F due to the strain of the measurement target 50 is applied to the optical fiber sensor 20 in the reference state in the direction in which the base 22 is contracted, the arm portions 25 and 26 are as shown in FIG. ), The tension with respect to the grating portion 1a of the first sensor optical fiber 1 is decreased to Te−ΔTe, and the Bragg wavelength is shortened according to the decrease ΔTe, and the second The tension on the grating portion 1a of the sensor optical fiber 1 'increases to Te + ΔTe, and the Bragg wavelength becomes longer according to the increase ΔTe.

  On the other hand, when the external force F due to the strain of the measurement target 50 is not applied and there is a temperature change ΔT, for example, as shown in FIG. The entire body 21 expands integrally with the object 50 to be measured, the distance between the arm portions 25 and 26 increases while maintaining the reference state, and the tensions of the optical fibers 1 and 1 ′ for both sensors increase to Te + ΔTe ′. The Bragg wavelength becomes longer according to the increment ΔTe ′.

  On the contrary, as shown in FIG. 5B, when the measurement target 50 is contracted, the entire optical fiber sensor 20 having the same linear expansion coefficient as that of the measurement target 50 is contracted in the same manner as the measurement target 50, and the arm portions 25 and 26 remain in the reference state. And the tensions of the optical fibers 1 and 1 'for both sensors are reduced to Te-ΔTe', and the Bragg wavelength is shortened according to the decrease ΔTe '.

In the optical fiber sensor 20 having the above structure, when an external force accompanying the strain ε of the measurement object 50 and a temperature change ΔT are simultaneously loaded, the shift amount Δλall of the reflected light wavelength generated in the first sensor optical fiber 1 _{(+ )} And the shift amount Δλall ₍₋₎ of the reflected light wavelength generated in the second optical fiber for sensor 1 ′ is expressed by the following equations (31) and (32) using the parameters Δλ _{B2 to} Δλ _B5 described above. These are respectively expressed and detected by the wavelength variation detecting means 43.

Δλall ₍₊₎ = Δλ _B2 + Δλ _B3 + Δλ _B4 + Δλ _B5 (31)
Δλall ₍₋₎ = Δλ _B2 + Δλ _B3 −Δλ _B4 −Δλ _B5 (32)

By distortion component calculation means 44, the shift amount of the difference [Delta] [lambda] _dif is obtained by calculating the following equation (33).

Δλ _dif = Δλall ₍₊₎ −Δλall ₍₋₎ = 2Δλ _B4 + 2Δλ _B5
= 2λ _B η (1−ρ _C ) (γ−β) ΔT + 2λ _B η (1−ρ _C ) ε (33)

Here, since the thermal expansion coefficient β of the sensor support 21 is equal to the thermal expansion coefficient γ of the measurement target 50 (Δλ _B4 = 0), the first item of the above equation (33) is 0, and the shift amount difference Δλ _dif is expressed by the following equation (33) ′.

Δλ _dif = 2λ _B η (1-ρ _C ) ε (33) ′

Therefore, by storing the Bragg wavelength λ _B , the strain magnification ratio η, and the photoelastic coefficient ρ _{C serving} as a measurement reference in advance in the calculation unit 42, the measurement target 50 can be obtained from the shift amount difference Δλ _dif obtained by the measurement. Can be obtained by the following equation (34).

ε = Δλ _dif / [2λ _B η (1-ρ _C )] (34)

  Note that when the shift amount is added as in the following equation (35), a value dependent only on the temperature change ΔT is obtained by removing the influence of the distortion of the measurement target 50.

Δλall ₍₊₎ + Δλall ₍₋₎ = 2Δλ _B2 + 2Δλ _B3
= 2 (λ _B ζΔT + λ _B αΔT) + 2λ _B η (1−ρ _C ) (β−α) ΔT (35)

  Therefore, it is possible to calculate the temperature change ΔT from this equation, and in the above-described strain calculation, for example, an error due to a temperature change caused by a slight expansion coefficient difference can be further minimized.

  In the above embodiment, the optical fibers 1 and 1 'for both sensors are fixed to the surfaces of the arm portions 25 and 26. However, this does not limit the present invention, and as shown in FIG. 26 may be fixed in a state where the fiber is passed through holes 45, 46, 47, and 48 that pass through the center of the upper side and the lower side of 26.

  Further, as shown in FIG. 7, one sensor optical fiber 1 is fixed to a slit 70 continuing from the base 22 side through the hinges 23, 24 to the lower center of the arm portions 25, 26, and the upper surface of the arm portion 25. The other sensor optical fiber 1 ′ may be fixed to a slit 71 continuing from the upper center to the upper center and a slit 72 continuing from the upper surface of the arm portion 26 to the upper center.

Configuration diagram of an embodiment of the present invention The figure for demonstrating the equilibrium conditions of embodiment Operation explanatory diagram of the embodiment Operation explanatory diagram of the embodiment Operation explanatory diagram of the embodiment The figure which shows other embodiment of this invention The figure which shows other embodiment of this invention Schematic configuration diagram of conventional equipment Schematic configuration diagram of conventional equipment Schematic configuration diagram of conventional equipment

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1, 1 '... Optical fiber for sensors, 1a ... Grating part, 20 ... Optical fiber sensor, 21 ... Fiber support, 22 ... Base, 23, 24 ... Hinge, 25, 26 ... Arm part , 27, 28... Hinge, 29... Connecting part, 40... Measuring device, 41... Reflected light wavelength detecting means, 42 .. calculating part, 43. Means, 45-48 ... hole, 50 ... measuring object, 70-72 ... slit

Claims (2)

A base (22) fixed to a measurement object, and a pair of arms (25, 26) standing upright so as to face each other via hinges (23, 24) which are elastically deformable on one side of the base; And a connecting portion (29) for connecting intermediate positions of the pair of arm portions via elastically deformable hinges (27, 28), and is integrally formed of a material having an expansion coefficient equal to that of the measurement object. And a fiber support body (21) that receives a force for expanding and contracting the base from the measurement target and tilts the pair of arm portions in opposite directions,
A state in which a grating part (1a) having a fiber Bragg grating is formed, and the grating part is sandwiched between positions of one end side of the pair of arm parts of the fiber support that are separated from the connecting part by a predetermined distance; A first optical fiber for sensor (1) fixed to the arm with tension at
A grating portion (1a) in which a fiber Bragg grating is formed, and the first arm portion of the fiber support body is positioned between the other end side of the predetermined distance away from the coupling portion; The tilting of the pair of arm portions due to the tension of the sensor optical fiber is restricted, and is fixed to the arm portion in a state where a tensile force for balancing the tension is applied, and is the same as the first sensor optical fiber An optical fiber sensor comprising a second optical fiber for sensor (1 ') having characteristics. An optical fiber sensor (20) according to claim 1;
Reflected light wavelength detecting means (41) for detecting the wavelength of the reflected light by the respective grating portions by making light incident on the first sensor optical fiber and the second sensor optical fiber of the optical fiber sensor;
Wavelength change amount detecting means for detecting a change amount of the reflected light wavelength of the first sensor optical fiber during a predetermined period and a change amount of the reflected light wavelength of the second sensor optical fiber during the predetermined period, respectively. 43)
The amount of change in wavelength associated with the distortion of the measurement object by subtracting the amount of change in wavelength of the first optical fiber for sensor detected by the wavelength change amount detection means from the amount of change in wavelength of the optical fiber for second sensor. And a distortion component calculating means (44) for calculating

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